1. Field of the Invention
This invention relates to the separation of one substance from another based on their differing magnetic susceptibilities.
2. The Prior Art
Magnetic separations are, of course, widely employed for many different applications, using many different specific techniques. In theory, the interaction of a substance with a magnetic field determines whether it is defined as a paramagnetic or diamagnetic substance. Paramagnetic substances concentrate or draw within themselves lines of magnetic force and achieve a flux density greater than that found in the surrounding field in a vacuum. In a magnetic field the dipole (an opposing north-south pole pair within a substance) of a paramagnetic substance will experience a torque, i.e., a rotational force, which tends to align the dipole parallel to the magnetic lines of force. If the field is nonuniform, that is, one where the field intensity or flux density varies with distance, then what is known as a tractive force will be exerted in the direction of increasing field strength.
For diamagnetic substances in a magnetic field, the interactions are the opposite. The magnetic lines of force are diverged within the substance; thus, it contains a flux density less than the surrounding field. The rotational force tends to align the substance perpendicular to the magnetic lines of force. The tractive force is in a direction toward decreasing field strength.
The rotational force or torque exerted on a substance in a magnetic field is a product of the pole strength, the distance between poles and the field strength. The tractive force is a product of the mass of the particle, its magnetic susceptibility, the field strength, and the magnetic gradient. The tractive force for ferromagnetics, an important subclass of paramagnetics, follows the above guidelines; however, the magnetic susceptibility of ferromagnetic materials is a complex function of the field strength.
Magnetic separation, which is widely used to beneficiate ores divides the feed material into at least two products: more magnetic, sometimes called "magnetic", and less magnetic, sometimes called "nonmagnetic". In today's commerce, only paramagnetic substances are removed from the feed into the "magnetic" fraction, and the primary force used to withdraw this "magnetic" fraction is the tractive force.
For a separation of "magnetics" from "nonmagnetics", feed material is transported through a nonuniform magnetic field, i.e., a field with a gradient. There, the magnetic separation of discrete particles occurs based upon a three-way competition between the magnetic; gravitational, frictional, hydraulic, inertial, and other forces; and interparticle forces.
The magnetic force (tractive) to move a susceptible particle away from other particles can be increased by increasing the field strength or increasing the field gradient. The magnetic fields of magnetic separators of low intensity (a few kilogauss or less) usually are produced by permanent magnets; high intensity fields up to 20 kilogauss (sometimes greater with superconducting systems) are produced by electromagnets. Low gradients result from specially shaped pole pieces. High gradients are obtained by packing a matrix of filamentary ferromagnetic material in a magnetized volume; high field gradients result at the edges of the filaments.
Interparticle forces may be magnetic in origin. They arise because a paramagnetic particle in a magnetic field concentrates lines of force; it therefore acts as a magnet with respect to a second susceptible particle because it converges the field. The net effect is the aggregation or flocculation of magnetic particles, particularly if the particles are small, if the magnetic susceptibility is large, and if the field is intense.
There are several types of magnetic separators used for beneficiating ores, including, for example, drum separators, induced roll separators, and crossbelt separators. Drum separators contain stationary electromagnets or permanent magnets inside rotating drums. Feed material is introduced onto the top of the drum. The magnetic force holds the magnetic particles to the drum until rotation of the drum carries the magnetics past the point where nonmagnetics fall away from the drum. The magnetics are released when they pass the point where the magnet ends. With this type of separator, the complete separation must occur in the short time it takes for the particles to pass over about 90.degree. arc of the drum. Induced roll separators acquire the magnetic force on the roll through induction from a large electromagnet. The roll is comprised of laminae alternately of magnetically permeable and impermeable material. As a result, the permeable laminae acquire an induced field, and a gradient develops between adjacent permeable laminae which then produces a tractive force upon susceptible particles passing over the roll. Nonmagnetics fall free from the roll; magnetics are retained upon the roll until brushed free on the back side of the roll. Thus the induced roll, like the drum separator, can provide only a short time for separation. However, for the induced roll the time is even shorter since induced rolls are typically five inches in diameter, while drum separators are typically 36 inches in diameter. The crossbelt separator consists of a main feed belt which passes beneath one or two magnets with a strongly converging field. Susceptible particles are drawn up toward the magnet but intercepted on the bottom of a crossbelt moving perpendicularly to the feed belt and located below the top pole of the magnet but above the feed particles. This crossbelt removes magnetics from the feed material and the magnets of the separator. All of the separation between particles must occur in the short time it takes for a particle on the feed belt to pass beneath the crossbelt, and although two crossbelts are frequently employed, the requirement of having a practical throughput demands a fast moving feed belt, hence a short time in the magnetic field.
Several patents discuss more specific techniques for accomplishing magnetic separations, including U.S. Pat. No. 3,725,262. The process disclosed in this reference sets forth a technique for reclaiming a contaminated liquid-solid mixture, which in part includes a magnetic separation over a region which contains a series of bar magnets arranged in a pattern to permit the magnetic particles to adhere to the regions of strongest magnetic intensity. In a process for separating nonferromagnetic, conductive metals from nonferromagnetic, nonconductive materials, U.S. Pat. No. 4,003,830 discloses in part the use of a herringbone-shaped pattern of magnets with alternating polarities arranged such that eddy currents are produced in the magnetic field. The result is a rather complex force field which affects the conductive nonmagnetic portion of the material being processed in a manner such that it is driven in an opposite direction to the flow of the resulting material.
A major disadvantage of many such magnetic separators is the short time available for the magnetic force to act upon and separate the more magnetic particles. As is discussed by Gaudin in Principles of Mineral Dressing, McGraw-Hill Book Co., N.Y., N.Y., 1939, p. 441, if the operation of a magnetic separator is to be continuous, the process must be carried out on a stream of particles passing into and through a magnetic field. The duration of application of the field is therefore limited to a short time, usually a fraction of one second. Gaudin comments further that by reducing the rate of passage of particles, it is possible to affect less susceptible particles. Information on this time effect, and additionally the effect of preliminary magnetization, is presented in U.S. Bureau of Mines R.I. 6411, wherein it is pointed out that the magnetic tractive force on particles varies not only in accordance with the static susceptibility but also with respect to magnetization in a preliminary field and time of retention in the tractive field.
The problem of entrapment of nonmagnetics in magnetics is also a major disadvantage to an effective magnetic separation. The phenomenon is addressed by Gaudin, supra, p. 441, wherein it is pointed out that occlusion of nonmagnetic material within magnetic flocs is especially serious in dry separators working on fine, highly magnetic material. It is further stated that this is caused in part by a sudden rush of the highly magnetic material toward the zone in the field that has the highest density, and in part by the instant formation of chains and flocs of magnetized particles which occlude and entrap nonmagnetic particles.
The process of the present invention obviates these disadvantages permitting the material being treated to be acted upon in the magnetic field for virtually an indefinite period of time. Furthermore, the magnetic force exerted by the magnetic field arrangement of the invention produces a tumbling effect as a result of the realignment of the magnetic particles to alternating opposing particles, thereby freeing the nonmagnetic particles from entrapment by magnetic particles.